The field of micromachined fluidic devices encompasses all systems that process components (e.g., gases, liquids, solid particles (e.g., beads), complex molecules (e.g., DNA), and mixtures thereof) and contain small features (minimum feature sizes smaller than 500 μm ). Such micromachined devices have been shown to be useful in many fields, including chemical and biological analyses (e.g., capillary electrophoretic separations), small-scale chemical synthesis, and measurement of reaction kinetics. The smaller dimensions inherent in micromachined devices enable significantly smaller fluid flow rates, reduced system size, and, in many cases, improved performance.
A variety of fluid-processing micromachined devices are disclosed in the prior art. For example, U.S. Pat. No. 6,192,596 provides an active microchannel fluid processing unit and a method of making the unit. Arrays of parallel microchannels separated by thermally conductive fins provide the mechanism for heat transfer to or from fluids moving through the microchannels.
U.S. Pat. No. 6,193,501 discloses a microcombustor of sub-millimeter dimensions. A preferred embodiment includes a wafer stack of at least three wafers, with the central wafer housing a combustion chamber. At least one inlet and one outlet are included for the insertion of reactants and the exhaust of a flame.
U.S. Pat. No. 4,516,632 discloses a microchannel crossflow fluid heat exchanger and a method for making the same. The heat exchanger is formed from a stack of thin metal sheets bonded together.
One possible application of fluid-processing micromachined devices is as portable electric generators. This application is promising because the energy density of chemical fuels exceeds that of presently available batteries by approximately two orders of magnitude. However, to take advantage of the high energy density of chemical fuels and compete with batteries for portable power applications, suitable efficient designs for micro-sized fuel processors/generators that convert chemical to electrical energy must be created. Fuel processors/generators usually require regions of high temperature in order to sustain the desired reaction. The power consumed in sustaining those temperatures reduces the overall efficiency of the system. As device dimensions become smaller, it becomes increasingly difficult to maintain the thermal gradients and thermal insulation required for efficient fuel processing. Often with existing micromachined fuel processing devices, significantly more power is required to maintain the temperature within the high temperature regions of the device than is contained in the fuel, precluding the device's use for portable power generation.
Thermal management is crucial to producing efficient devices designed to operate with separate features held at different temperatures. In particular, thermal isolation of the reaction zone in micromachined power generation systems is paramount. For micromachined non-fluidic devices, thermal management is achieved by simple thermal insulation using long, thin and/or non-conductive supports, often assisted by packaging in vacuum. Examples of non-fluidic micromachined devices providing thermal management include, for example, bolometers, such as those disclosed in U.S. Pat. Nos. 5,021,663 and 5,789,753. However, the present inventors have concluded that micromachined fluidic devices add three unique difficulties: the need for enclosed fluidic structures connecting the thermal regions; the potential of added heat flow through thermal convection; and, frequently, a desire to include regions where the walls of the fluid conducting tube are held isothermal. Thus, a successful thermal management scheme for a micromachined fluidic device must include, in addition to certain known inventions and techniques used in non-fluidic thermal devices, a means to provide fluid communication with high temperature regions without causing excessive heat flow either through the static structure or through the moving fluid. It is frequently desirable to include in this scheme means to ensure thermal uniformity in specific regions of the device.
Accordingly, it would be advantageous to provide a micromachined device capable of efficiently conducting a chemical process involving at least one fluid, wherein a high temperature reaction zone is thermally isolated from its environment. It also would be advantageous to provide a micromachined device for conducting a chemical reaction involving fluidic reactants and wherein operation of the device consumes substantially less energy than can be produced from the fluid reactants. Such a device could be used as part of a portable electric generator.